Hydraulic circuit in work vehicle

ABSTRACT

A hydraulic circuit includes a control valve that controls flow of pressure oil from a hydraulic source to work hydraulic cylinders, an operating device that issues a command for drive of the control valve, valve devices each comprising a check valve, each provided in correspondence to one of the plurality of work hydraulic cylinders to allow and prohibit outflow of pressure oil from a work hydraulic cylinder, a commanding device that outputs a command allowing or a command prohibiting extension/contraction for each of the work hydraulic cylinders and a control device that controls each of the valve devices to allow outflow of pressure oil from the work hydraulic cylinder by invalidating a check valve function in response to the command for allowing extension/contraction and to prohibit outflow of pressure oil from the work hydraulic cylinder with the check valve in response to the command for prohibiting extension/contraction.

TECHNICAL FIELD

The present invention relates to a hydraulic circuit in a work vehicle,which drives an outrigger cylinder, a blade cylinder or the likedisposed at an undercarriage of a rotatable work vehicle such as a wheelhydraulic excavator.

BACKGROUND ART

Hydraulic circuits used for outrigger cylinder drive in the related artinclude, for instance, the hydraulic circuit disclosed in Japanese LaidOpen Utility Model Publication No. S63-4772.

In conjunction with the hydraulic circuit disclosed in this publication,the bottom chambers or the rod chambers of outrigger cylinders disposedto the front, the rear, the left side and the right side of the vehicleare individually made to communicate via hydraulic pilot switchingvalves. In response to a switching operation at the switching valve,pressure oil is allowed to flow to a desired hydraulic cylinder whilecutting off the flow of pressure oil to the other hydraulic cylinders.This system makes it possible to operate the outriggers on the frontside, the rear side, the left side and the right side independently ofone another.

However, if high pressure oil is applied to hydraulic cylinders in thecircuit disclosed in the publication described above, in which the oilflow is cut off with the switching valve, the oil may leak from theswitching valve and in such a case, it may not be possible to hold thevehicle body in a jacked up state. While a leakless switching valve maybe utilized to avoid this problem, the use of the leakless switchingvalve is bound to be costly.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a drive circuit for awork hydraulic cylinder, achieving a structure capable of maintaining anextension/contraction state of the hydraulic cylinder at low cost.

A hydraulic circuit in a work vehicle according to the present inventionincludes an undercarriage, a revolving superstructure rotatably mountedatop the undercarriage, a hydraulic source disposed at the revolvingsuperstructure, at least a plurality of work hydraulic cylindersdisposed at the undercarriage, that are to be driven by pressure oilfrom the hydraulic source, a control valve that controls flow ofpressure oil from the hydraulic source to the work hydraulic cylinders,an operating means for issuing a command for drive of the control valve,valve devices each comprising a check valve, each provided incorrespondence to one of the plurality of work hydraulic cylinders toallow and prohibit outflow of pressure oil from a work hydrauliccylinder, a commanding means for outputting one of a command forallowing extension/contraction and a command for prohibitingextension/contraction for each of the work hydraulic cylinders, and acontrol means for controlling each of the valve devices so as to allowoutflow of pressure oil from the work hydraulic cylinder by invalidatinga check valve function thereof in response to the command for allowingextension/contraction output from the commanding means and so as toprohibit outflow of pressure oil from the work hydraulic cylinder withthe check valve in response to the command for prohibitingextension/contraction output by the commanding means.

In this manner, leakage of pressure oil from the hydraulic cylinder canbe prevented and an extension/contraction state of the hydrauliccylinder can be maintained at low cost.

The hydraulic circuit may be formed so that oil flows between theundercarriage and the revolving superstructure via a pair of pipelinesand that the pair of pipelines are branched in the undercarriage toconnect with each of the work hydraulic cylinders.

The valve devices may be constituted as pilot-operated check valvescontrolled by a pilot pressure. In this case, it is preferable that apilot hydraulic circuit is formed so as to guide the pilot pressure fromthe revolving superstructure to the undercarriage via a single pilotpipeline and so as to branch the pilot pipeline in the undercarriage toconnect with each of the valve devices.

The valve devices may also be constituted as solenoid controlleddirectional control valves, each comprising a check valve.

Outflow of pressure oil from the work hydraulic cylinders may be allowedif the command for allowing extension/contraction is output from thecommanding means and the operation of the operating means is detectedwith the detection means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view of a wheel hydraulic excavator in which thepresent invention is adopted;

FIG. 2 is an enlargement of an essential portion of FIG. 1;

FIG. 3 is a hydraulic circuit diagram pertaining to a first embodimentof the present invention;

FIG. 4 shows a relay circuit that controls solenoid controlleddirectional control valves in FIG. 3;

FIG. 5 shows an operating member that outputs control commands for thesolenoid controlled directional control valves;

FIG. 6 is a hydraulic circuit diagram pertaining to a second embodimentof the present invention; and

FIG. 7 shows a relay circuit that controls the solenoid controlleddirectional control valves in FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

The following is an explanation of the first embodiment achieved byadopting a hydraulic circuit according to the present invention in awheel hydraulic excavator, given in reference to FIGS. 1 to 5.

As shown in FIG. 1, the wheel hydraulic excavator includes anundercarriage 1 and a revolving superstructure or revolvingupperstructure 2 rotatably mounted atop the undercarriage 1. An operatorcab 3 and a work front attachment 4 constituted with a boom 4 a, an arm4 b and a bucket 4 c are disposed at the revolving superstructure 2. Theboom 4 a is hoisted as a boom cylinder 4 d is driven, the arm 4 b ishoisted as an arm cylinder 4 e is driven and the bucket 4 c is engagedin a lift operation or a dump operation as a bucket cylinder 4 f isdriven. A traveling motor 5, which is hydraulically driven, is disposedat the undercarriage 1, and the rotation of the traveling motor 5 istransmitted to wheels 6 (tires) via a drive shaft and axles.

As shown in FIG. 2, an outrigger 10 is disposed near each of the tires 6disposed at the front and the rear of the undercarriage 1 on the leftand right sides. An outrigger cylinder 11 is attached to the outrigger10 and as the cylinder 11 extends and contracts, the outrigger 10rotates with a hinge pin 10 a acting as its fulcrum. As the cylinder 11extends, the outrigger 10 is lowered to the ground to lift the vehicleoff the ground (jack up), and as the cylinder 11 contracts and retracts,the outrigger 10 is stored into the undercarriage 1, thereby loweringthe vehicle onto the ground (jack down).

FIG. 3 is a hydraulic circuit diagram pertaining to the first embodimentof the present invention showing a drive circuit for the outriggercylinders 11 as its main feature. It is to be noted that referencenumerals 11FL, 11FR, 11RL and 11RR respectively indicate the outriggercylinders 11 at the front left, the front right, the rear left and therear right of the vehicle.

In the circuit shown in FIG. 3, the pressure oil from a hydraulic pump21 disposed at the revolving superstructure 2 travels through a centerjoint 25 via a directional control valve 22 and a pipeline 23 or 24 andis guided to the undercarriage. The oil returning from the undercarriage1 travels through the center joint 25 via the pipeline 24 or 23 and isguided to a reservoir.

The directional control valve 22 is switched in response to an operationof an operation lever 26. Namely, as the operation lever 26 is operated,a pressure reducing valve 27 is driven in correspondence to the extentto which the operation lever is operated and a pilot pressure from ahydraulic source 28 is applied to a pilot port at the directionalcontrol valve 22 via a pilot pipeline 29 or 30, thereby switching thedirectional control valve 22. A shuttle valve 31 is disposed between thepilot pipelines 29 and 30, and the pilot pressure generated at therevolving superstructure 2 is guided to the undercarriage 1 afterpassing through the center joint 25 via the shuttle valve 31 and a pilotpipeline 32.

Pilot-operated check valves 12 a and 12 b are respectively disposed onthe intake side of a bottom chamber 11 a and a rod chamber 11 b of eachof the outrigger cylinders 11FL, 11FR, 11RL and 11RR. The bottomchambers 11 a communicate with one another via the pilot-operated checkvalves 12 a and they also connect with the pipeline 23. The rod chambers11 b communicate with one another via the pilot-operated check valves 12b and they also connect with the pipeline 24.

The pilot-operated check valves 12 a and 12 b are controlled by a pilotpressure supplied from the outside. Pilot ports of the pilot-operatedcheck valves 12 a and 12 b are connected to the pilot pipeline 32 viasolenoid controlled directional control valves 34 to 37 provided incorrespondence to the outrigger cylinders 11FL, 11FR, 11RL and 11RRrespectively. Solenoids 34 a to 37 a of the solenoid controlleddirectional control valves 34 to 37 are excited or demagnetized inresponse to electrical signals output via, for instance, slip-rings fromthe revolving superstructure 2.

As the solenoids 34 a to 37 a become excited, the respective solenoidcontrolled directional control valves 34 to 37 are each switched to aposition “a”, and, as a result, the pilot pressure from the pilotpipeline 32 is applied to the pilot-operated check valves 12 a and 12 b.This invalidates the function of the pilot-operated check valves 12 aand 12 b as check valves and the pilot-operated check valves 12 a and 12b are allowed to function simply as open valves, thereby allowing thepressure oil to flow out from the bottom chambers 11 a and the rodchambers 11 b.

As the solenoids 34 a to 37 a become demagnetized, the respectivesolenoid controlled directional control valves 34 to 37 are eachswitched to a position “b”, thereby stopping the supply of the pilotpressure to the pilot-operated check valves 12 a and 12 b. As a result,the pilot-operated check valves 12 a and 12 b function as check valvesand the flow of pressure oil out of the bottom chambers 11 a and the rodchambers 11 b becomes prohibited. Since the pilot-operated check valves12 a and 12 b all adopt a structure having a poppet valve which becomespressed against the surface of the main unit seat by the pressuregenerated in a reverse flow instead of a structure having a spool thatmoves within a valve unit as in a switching valve, hardly any leakoccurs and the cost of such pilot-operated check valves can be kept low.

FIG. 4 shows a relay circuit that controls the power supply to thesolenoids 34 a to 37 a. This relay circuit is switched in response tooperations of, for instance, a dial-type front/rear selector switch 41and a dial-type left/right selector switch 42 shown in FIG. 5. Theswitches 41 and 42 are installed in the operator's cab 3.

As shown in FIG. 5, the front/rear selector switch 41 can be operated toan OFF position, an F position, an A position or an R position toselectively operate the outrigger cylinders 11FL and 11FR on the frontside and the outrigger cylinders 11RL and 11RR on the rear side. Namely,the switch 41 is operated to the F position to drive the front-sidecylinders 11FL and 11FR, is operated to the R position to drive therear-side cylinders 11RL and 11RR, is operated to the A position todrive the cylinders 11FL, 11FR, 11RL and 11RR on both the front side andthe rear side and is operated to the OFF position if none of thecylinders 11FL, 11FR, 11RL and 11RR is to be driven.

The left/right selector switch 42, which can be operated to an Lposition, an A position or an R position, is used to selectively operatethe outrigger cylinders 11FL and 11RL and the outrigger cylinders 11FRand 11RR on the left side and the right side. Namely, the switch 42 isoperated to the L position to drive the left-side cylinders 11FL and11RL, is operated to the R position to drive the right-side cylinders11FR and 11RR and is operated to the A position to drive the cylinders11FL, 11FR, 11RL and 11RR on both the left side and the right side.

Through the switch operations described above, an allowextension/contraction command or a prohibit extension/contractioncommand is output to each of the outrigger cylinders 11FL, 11FR, 11RLand 11RR.

The relay circuit in FIG. 4 is now explained. If the front/rear selectorswitch 41 in FIG. 4 is operated to the OFF position, no power issupplied to coils at relays 43 and 44 and, as a result, the relays 43and 44 are each switched to a contact point “a”. Consequently, thesolenoids 34 a to 37 a are all demagnetized. As the front/rear selectorswitch 41 is operated to the F position, terminals 1 and 2 at the switch41 come into communication with each other as shown in the figure andpower is thus supplied to the coil at the relay 43 thereby switching therelay 43 to a contact point “b”. As the front/rear selector switch 41 isoperated to the R position, switch terminals 4 and 5 come intocommunication with each other and thus, power is supplied to the coil atthe relay 44 to switch the relay 44 to a contact point “b”. As theselector switch 41 is operated to the A position, the switch terminals1, 3 and 4 come into communication with one another, and power is thussupplied to the coils at the relays 43 and 44, thereby switching boththe relays 43 and 44 to their contact points “b”.

If the left/right selector switch 42 is operated to the L position afterthe relay 43 is switched to the contact point “b”, terminals 1 and 2 atthe switch 42 come into communication with each other, as shown in thefigure, power is supplied to a coil at a relay 45, thereby switching therelay 45 to a contact point “b”. As a result, the solenoid 34 a becomesexcited. If the left/right selector switch 42 is operated to the Rposition, switch terminals 4 and 5 come into communication with eachother and power is thus supplied to a coil at a relay 46, therebyswitching the relay 46 to a contact point “b”. Consequently, thesolenoid 35 a becomes excited. If the left/right selector switch 42 isoperated to the A position, the switch terminals 1, 3 and 4 come intocommunication with one another and power is thus supplied to the coilsat the relays 45 and 46, thereby switching both the relays 45 and 46 totheir contact points “b”. As a result, the solenoids 36 a and 37 a areboth excited.

If the left/right selector switch 42 is operated to the L position afterthe relay 44 is switched to the contact point “b”, the switch terminals1 and 2 come into communication with each other and power is supplied toa coil at a relay 47, thereby switching the relay 47 to a contact point“b”. As a result the solenoid 36 a becomes excited. If the left/rightselector switch 42 is operated to the R position, the switch terminals 4and 5 come into communication with each other and power is supplied to acoil at a relay 48, thereby switching the relay 48 to a contact point“b”. Consequently, the solenoid 37 a becomes excited. If the left/rightselector switch 42 is operated to the A position, the switch terminals1, 3 and 4 come into communication with one another and power is thussupplied to the coils at the relays 47 and 48, thereby switching boththe relays 47 and 48 to their contact points “b”. As a result, thesolenoid 36 a and 37 a become excited.

The operation that characterizes the hydraulic circuit achieved in thefirst embodiment is now explained.

When the vehicle body is not to be jacked up or down (here afterreferred to as jack up/down) the front/rear selector switch 41 isoperated to the OFF position. In response to this switch operation, acommand for prohibiting extension or contraction of all the outriggercylinders 11 is output, and the solenoids 34 a to 37 a are alldemagnetized as described earlier, thereby switching the individualsolenoid controlled directional control valves 34 to 37 to the position“b”. As a result, the communication of the pilot-operated check valves12 a and 12 b with the pilot pipeline 32 becomes cut off, and thepilot-operated check valves 12 a and 12 b, with no pilot pressuresupplied thereto, function as check valves. In this state, even if thedirectional control valve 22 is switched and pressure oil is guided fromthe hydraulic pump 21 to the outrigger cylinders 11, the pressure oil isnot allowed to flow out of the bottom chambers 11 a and the rod chambers11 b. Thus, the cylinders 11 cannot be extended or contracted and thejack up/down of the vehicle body is prohibited.

In order to jack up/down the front of the vehicle body on the left sideand the right side, for instance, the front/rear selector switch 41 isoperated to the F position and the left/right selector switch 42 isoperated to the A position. In response to these switch operations, acommand for allowing extension and contraction of the outriggercylinders 11FL and 11FR and a command for prohibiting extension orcontraction of the outrigger cylinders 11RL and 11RR are output. As aresult, the solenoids 34 a and 35 a become excited, thereby switchingthe solenoid controlled directional control valves 34 and 35 to theposition “a”.

As the operation lever 26 currently at the neutral position is operatedin this state, the pilot pressure from the hydraulic source 28 isapplied to the pilot-operated check valves 12 a and 12 b of theoutrigger cylinders 11FL and 11FR via the pipeline 32, thereby enablingthe pilot-operated check valves 12 a and 12 b to function as openvalves. In addition, the pilot pressure from the hydraulic source 28 isalso applied to the directional control valve 22 to switch thedirectional control valve 22 to the position “a” or the position “b”. Inresponse, the pressure oil from the hydraulic pump 21 is guided to thebottom chambers 11 a or the rod chambers 11 b of the outrigger cylinders11FL and 11FR and the pressure oil is discharged from the rod chambers11 b or the bottom chambers 11 a. The front side outrigger cylinders11FL and 11FR can thus be engaged in operation simultaneously to jackup/down the front side of the vehicle body.

In order to jack up/down only either the left side or the right side(e.g., the left side) of the vehicle body at the front, the front/rearselector switch 41 is operated to the F position and also, theleft/right selector switch 42 is operated to the L position. In responseto these switch operations, a command for allowing extension andcontraction of the outrigger cylinder 11FL and a command for prohibitingextension or contraction of the outrigger cylinders 11FR, 11RL and 11RRare output. As a result, the solenoid 34 a becomes excited and thesolenoid controlled directional control valve 34 alone is switched tothe position “a”. As the operation lever 26 currently at the neutralposition is operated in this state, the pilot pressure is applied to thepilot-operated check valves 12 a and 12 b of the outrigger cylinder 11FLand thus the front-side cylinder 11FL alone is engaged in operationindependently of the others with the pressure oil supplied from thehydraulic pump 21.

In order to jack up/down the rear of the vehicle body on the left sideand the right side, the front/rear selector switch 41 is operated to theR position and the left/right selector switch 42 is operated to the Aposition. In response, the solenoids 36 a and 37 a become excited,thereby switching the solenoid controlled directional control valves 36and 37 to the position “a”. As the operation lever 26 currently at theneutral position is operated in this state, the pilot pressure isapplied to the pilot-operated check valves 12 a and 12 b of theoutrigger cylinders 11RL and 11RR to engage the rear side outriggercylinders 11RL and 11RR in operation at the same time, and thus, therear side of the vehicle body is jacked up/down.

In order to jack up/down only either the left side or the right side(e.g., the left side) of the vehicle body at the rear, the front/rearselector switch 41 is operated to the R position and also, theleft/right selector switch 42 is operated to the L position. Inresponse, the solenoid 36 a becomes excited and the solenoid controlleddirectional control valve 36 alone is switched to the position “a”. Asthe operation lever 26 currently at the neutral position is operated inthis state, the pilot pressure is applied to the pilot-operated checkvalves 12 a and 12 b of the outrigger cylinder 11RL and thus, therear-side cylinder 11RL alone is engaged in operation independently ofthe others with the pressure oil supplied from the hydraulic pump 21.

In order to jack up/down the left side or the right side of the vehiclebody both at the front and at the rear, the front/rear selector switch41 is operated to the A position and the left/right selector switch 42is operated to the L position or the R position. In response, thesolenoids 34 a and 36 a or the solenoids 35 a and 37 a become excited,thereby switching the solenoid controlled directional control valves 34and 36 or 35 and 37 to the position “a”. As the operation lever 26currently at the neutral position is operated in this state, the pilotpressure is applied to the pilot-operated check valves 12 a and 12 b ofthe outrigger cylinders 11FL and 11RL or the outrigger cylinders 11FRand 11RR to jack up/down the left side or the right side of the vehiclebody.

In order to jack up/down the entire vehicle body, the front/rearselector switch 41 is operated to the A position and the left/rightselector switch 42 is operated to the A position. In response, all thesolenoids 34 a to 37 a become excited, thereby switching the solenoidcontrolled directional control valves 34 to 37 to the position “a”. Asthe operation lever 26 is operated in this state, the pilot pressure isapplied to the pilot-operated check valves 12 a and 12 b of theoutrigger cylinders 11FL, 11FR, 11RL and 11RR to jack up/down the entirevehicle body.

The following advantages can be achieved in the first embodiment.

-   (1) The pilot-operated check valves 12 a and 12 b are disposed on    the intake side of the bottom chamber 11 a and the rod chamber 11 b    of each of the outrigger cylinders 11FL, 11FR, 11RL and 11RR, and    the solenoid controlled directional control valves 34 to 37 are    switched in response to switch operations to apply the pilot    pressure to the corresponding pilot-operated check valves 12 a and    12 b. Thus, the individual outrigger cylinders 11FL, 11FR, 11RL and    11RR are allowed to be engaged in operation independently of one    another and the vehicle body can be jacked up/down in any desired    manner. In addition, a structure in which leakage of pressure oil    from the outrigger cylinders 11 can be prevented and a specific    jacked-up state can be maintained is achieved at low cost.-   (2) The pressure oil from the hydraulic pump 21 is guided to the    undercarriage 1 via a pair of pipelines 23 and 24 which are branched    on the side where the undercarriage 1 is located to individually    connect with the outrigger cylinders 11FL, 11FR, 11RL and 11RR. This    makes it possible to reduce the number of high-pressure pipings    passing through the center joint 25, which, in turn, makes it    possible to miniaturize the center joint 25.-   (3) The flows of pressure oil to the outrigger cylinders 11FL, 11FR,    11RL and 11RR can be individually controlled with the single    operation lever 26 and the directional control valve 22 alone and    thus, the number of required parts can be reduced.-   (4) The pilot pressure from the hydraulic source 28 is guided to the    undercarriage 1 via the single pilot pipeline 32, and the pipeline    32 is branched on the side where the undercarriage 1 is located to    individually connect with the pilot-operated check valves 12 a and    12 b. Thus, the number of pilot pipings passing through the center    joint 25 can be reduced and the center joint 25 can be miniaturized.-   (5) The pilot pressure is supplied to the directional control valve    22 and the pilot-operated check valves 12 a and 12 b by operating    the operation lever 26 and thus, the pilot-operated check valves 12    a and 12 b are engaged in operation by interlocking with the    operation of the operation lever 26. As a result, any undesirable    movement of the outrigger cylinders 11 immediately after the    solenoid controlled directional control valves 34 to 37 are switched    in response to switch operations is prohibited to improve the    reliability of the outriggers 10.

Second Embodiment

The second embodiment of the present invention is explained in referenceto FIGS. 6 and 7.

While the pilot-operated check valves 12 a and 12 b are disposed on theintake side of the oil chambers 11 a and 11 b of the outrigger cylinders11 and their function as check valves is invalidated by the pilotpressure from the revolving superstructure 2 in the first embodiment,the function as check valves is invalidated by an electrical signaloriginating from the revolving superstructure 2 in the secondembodiment.

FIG. 6 is a hydraulic circuit diagram pertaining to the secondembodiment of the present invention, showing a drive circuit for theoutrigger cylinders 11 as its main feature. It is to be noted that thesame reference numerals are assigned to components identical to those inFIG. 3 and the following explanation focuses on differentiatingfeatures.

Solenoid controlled directional control valves 61 to 64, instead of thepilot-operated check valves 12 a and 12 b, are disposed on the intakeside of the bottom chambers 11 a and the rod chambers 11 b of theindividual outrigger cylinders 11FL, 11FR, 11RL and 11RR respectively.Accordingly, no pilot pipeline passes through the center joint 25 unlikein the first embodiment, and the number of pipelines passing through thecenter joint 25 is smaller than that in the first embodiment for thisreason. A pressure switch 65 is connected to the shuttle valve 31. Thepressure switch 65 is turned on by pilot pressure generated in responseto an operation of the operation lever 26, and the operation of theoperation lever 26 is thus detected.

The solenoid controlled directional control valves 61 to 64 each includebuilt-in check valves 60 a and 60 b. As solenoids 61 a to 64 a of thesolenoid controlled directional control valves 61 to 64 in FIG. 6 becomeexcited, the solenoid controlled directional control valves 61 to 64 areeach switched to the position “a”. Under such circumstances, thesolenoid controlled directional control valves function simply as openvalves, allowing pressure oil to flow out from the bottom chambers 11 aand the rod chambers 11 b. As the solenoids 61 a to 64 a becomedemagnetized, the solenoid controlled directional control valves 61 to64 are each switched to the position “b”. In response, the outflow ofthe pressure oil from the bottom chambers 11 a and the rod chamber 11 bbecomes prohibited by the check valves 60 a and 60 b.

FIG. 7 shows a relay circuit that controls the power supply to thesolenoids 61 a to 64 a. It is to be noted that the same referencenumerals are assigned to components identical to those in FIG. 4 and thefollowing explanation focuses on differentiating features. As thepressure switch 65 in FIG. 7 is turned on, power is supplied to a coilof a relay 66, thereby switching the relay 66 to a contact point “b”.Thus, relays 43 to 48 are switched in response to operations of theswitches 41 and 42 to excite or demagnetize the solenoids 61 a to 64 aas in the first embodiment.

The operation characterizing the second embodiment is now explained.

When the operation lever 26 is set to the neutral position, the pressureswitch 65 is turned off and the relay 66 is switched to a contact point“a”. In this state, the solenoids 61 a to 64 a remain demagnetized atall times regardless of the positions of the switches 41 and 42. Thesolenoid controlled directional control valves 61 to 64 are thus allswitched to the position “b”, the outrigger cylinders 11 are notextended or contracted and the jack up/down operation of the vehiclebody is prohibited.

As the operation lever 26 currently at the neutral position is operated,the pressure switch 65 is turned on and the relay 66 is switched to thecontact point “b”. In this state, the solenoids 61 a to 64 a becomeexcited in response to operations of the switches 41 and 42 and thecorresponding solenoid controlled directional control valves 61 to 64are switched to the position “a”, as in the first embodiment. As aresult, the outrigger cylinders 11 are extended or contracted inresponse to an operation of the operation lever 26 to jack up/down thevehicle body.

As described above, the solenoid controlled directional control valves61 to 64 each having the check valves 60 a and 60 b are disposed on theintake side of the oil chambers 11 a and 11 b of the individualoutrigger cylinders 11FL, 11FR, 11RL and 11RR and the solenoidcontrolled directional control valves 61 to 64 are switched in responseto switch operations in the second embodiment. Thus, the drive of eachof the outrigger cylinders 11FL, 11FR, 11RL and 11RR can be enabled orprohibited independently and, at the same time, leakage of pressure oilfrom the cylinders 11 can be prevented in an inexpensive structure.Since no pilot pipeline needs to pass through the center joint 25, thecenter joint 25 can be further miniaturized. An operation at theoperation lever 26 is detected with the pressure switch 65, and if thedrive of an outrigger cylinder 11 is selected through a switch operationwhile the pressure switch 65 is in an on state, the correspondingsolenoid among the solenoids 61 a to 64 a is excited, thereby preventingany undesirable movement of the outrigger cylinders 11 when theoperation lever 26 is not operated.

It is to be noted that while the pilot pressure generated in response toan operation of the operation lever 26 is guided to the pilot pipeline32 via the shuttle valve 31 in the first embodiment, the operation ofthe operation lever 26 may be detected with a pressure sensor 65instead, as in the second embodiment, and the pilot pressure may beguided to the pilot pipeline 65 when the pressure switch 65 is in an onstate.

While the check valve function is invalidated by interlocking with anoperation of the operation lever 26 in the embodiments described above,the check valve function does not need to be invalidated by interlockingwith the operations of the operation lever 26 and instead, the checkvalve function may be invalidated simply in response to operations ofthe switches 41 and 42.

While an explanation is given above in reference to the embodiments on ahydraulic circuit that includes the outrigger cylinders 11FL, 11FR, 11RLand 11RR disposed on the left side and the right side of the vehiclebody at the front and the rear, the present invention may be adoptedequally effectively in a hydraulic circuit having outrigger cylindersonly either on the front side or the rear side of the vehicle body,e.g., outrigger cylinders 11RL and 11RR (rear side only) The presentinvention may also be adopted with equal effectiveness in conjunctionwith work hydraulic cylinders (e.g., blade cylinders), as well as inconjunction with the outrigger cylinders 11 disposed at theundercarriage 1.

A command for the drive of the directional control valve 22 may beissued through an operating member (e.g., a switch) other than theoperation lever 26. While the allow extension/contraction command andthe prohibit extension/contraction command are output through thedial-type switches 41 and 42, ON/OFF switches (e.g., toggle switches)may instead be provided in a quantity corresponding to the number ofoutrigger cylinders 11FL, 11FR, 11RL and 11RR and the allowextension/contraction command and the prohibit extension/contractioncommand may be output through operations of these switches.

While the power supply to the solenoids 34 a to 37 a or 61 a to 64 a iscontrolled with a relay circuit, signals originating from the operationlever 26 and the switches 41 and 42 may be taken into a computer toenable computer control. In other words, the control means may adopt astructure other than those explained in reference to the embodiments.

INDUSTRIAL APPLICABILITY

While an explanation is given above on an example in which the presentinvention is adopted in a wheel hydraulic excavator, the presentinvention may be adopted in other types of work vehicles includingconstruction machines such as wheel loaders and truck cranes, as well.It may also be adopted in conjunction with jack-up cylinders for largecranes.

1. A hydraulic circuit in a work vehicle, comprising: an undercarriage;a revolving superstructure rotatably mounted atop the undercarriage; ahydraulic source disposed at the revolving superstructure; at least aplurality of work hydraulic cylinders disposed at the undercarriage,that are to be driven by pressure oil from the hydraulic source; acontrol valve that controls flow of pressure oil from the hydraulicsource to the work hydraulic cylinders; an operating device that issuesa command for drive of the control valve; valve devices each comprisinga check valve, each provided in correspondence to one of the pluralityof work hydraulic cylinders to allow and prohibit outflow of pressureoil from a work hydraulic cylinder; a commanding device that outputs oneof a command for allowing extension/contraction and a command forprohibiting extension/contraction for each of the work hydrauliccylinders; a control device that controls each of the valve devices soas to allow outflow of pressure oil from the work hydraulic cylinder byinvalidating a check valve function thereof in response to the commandfor allowing extension/contraction output from the commanding device andan operation of the operating device, and so as to prohibit outflow ofpressure oil from the work hydraulic cylinder with the check valve inresponse to the command for prohibiting extension/contraction output bythe commanding device, wherein the control device also prohibits outflowof pressure oil from the work hydraulic cylinder with the check valvewhile the operating device is not operated.
 2. A hydraulic circuit in awork vehicle according to claim 1, wherein: the hydraulic circuit isformed so that oil flows between the undercarriage and the revolvingsuperstructure via a pair of pipelines through which drive pressure issupplied to the work hydraulic cylinders and the drive pressure is thenreturned and that the pair of pipelines are branched in theundercarriage to connect with each of the work hydraulic cylinders.
 3. Ahydraulic circuit in a work vehicle according to claim 1, wherein: thevalve devices are constituted as pilot-operated check valves controlledby a pilot pressure.
 4. A hydraulic circuit in a work vehicle accordingto claim 3, wherein: a pilot hydraulic circuit is formed so as to guidethe pilot pressure generated at the revolving superstructure in responseto an operation at the operating device to the undercarriage via asingle pilot pipeline and so as to branch the pilot pipeline in theundercarriage to connect with each of the valve devices.
 5. A hydrauliccircuit in a work vehicle according to claim 1, wherein the valvedevices are constituted as switching valves, each comprising a checkvalve, which is controlled by an electrical signal.